Ankle Radiographic Evaluation

Anatomy

The ankle joint is formed by the tibia, fibula, and talus. The medial malleolus is an osseous excrescence from the medial distal tibia. The posterior lip of the distal tibia is often referred to as the posterior malleolus. The lateral malleolus is the most distal extension of the distal fibula. The tibial plafond is the name given to the articular surface of the distal tibia. Fractures involving the medial and lateral malleoli are termed bimalleolar fractures. By extension, fractures of all three malleoli are referred to as trimalleolar fractures.

The talar anatomy is complex and is divided into the head, neck, and body. Because of its unique role in the foot and ankle mobility, it is extensively covered with articular cartilage, and there are few muscle or tendon attachments.[5] The talar head is covered with hyaline cartilage and articulates with the navicular bone at its anterior/distal aspect. The head also articulates with the calcaneus via the anterior facet at its inferior margin (anterior portion of the subtalar joint). The talar body articulates with the calcaneus inferiorly at the middle (anteromedial) and posterior (posterolateral) facets. Further, the middle facet articulates with the medial process from the calcaneus (also called the sustentaculum tali). The cephalad aspect of the body (talar dome or trochlea) articulates with the tibia at the tibiotalar joint. The neck is the portion that connects the head and body and does not possess an articular surface or cartilage. The inferior margin of the talar neck forms the superior margin of the sinus tarsi and tarsal canal.[5]

The posterior process of the talar body is composed of medial and lateral tubercles, which form a groove through which the flexor hallucis longus tendon runs. The lateral process extends from the lateral aspect of the talar body and articulates with the fibula superiorly, and forms the anterolateral portion of the posterior facet of the posterior subtalar joint.

Vascular supply to the ankle is complex and is derived from the three distal branches of the popliteal artery. A particular concern for vascular disruption should be paid to the talus due to its tenuous blood supply. This consideration arises because of the extensive articular cartilage coverage of the talus, which prevents direct vascular supply over much of the talar surface.

The ankle articulations are maintained by ligamentous support, including the lateral collateral ligaments, the deltoid (medial) ligaments, the syndesmotic ligaments, and the tibiofibular ligaments. Classically, these ligaments are injured in predictable sequences based on the pattern of injury. Additionally, avulsion fractions at the site of attachment of the ligament may occur due to these same patterns.

Ankle fractures are created by the movement of the talus within the ankle mortise, with leverage exerted by the foot. The primary motions of the ankle are external rotation, abduction, and adduction. Secondary motions include dorsiflexion and plantar flexion. Lines of force are directed against the lateral, medium, and posterior malleoli.

Many ankle fracture types and fracture patterns have been described. For the sake of brevity, we will only review the most common fracture types in this article. The most common ankle fracture involves the lateral malleolus, and the most common injury of this structure is an obliquely oriented fracture in the coronal plane. This fracture extends from the joint line anteroinferior to the posterior aspect of the fibula posterosuperiorly and arises from supination-external rotation injuries. These can be difficult to see on anteroposterior (AP) views while nondisplaced and are more easily seen on mortise and lateral views. Avulsion fractures from the distal tip of the lateral malleolus are also common and result from inversion injuries to the ankle. These are typically well evaluated on AP and lateral views. Other common fracture types involve transverse fractures at, distal, or proximal to the joint line, and these are due to pronation-external rotation forces. Transverse fractures invariably involve injury to the syndesmotic ligament.

Medial malleolus fractures are not uncommon, and the most common fractures are transversely oriented, distal to the corner of the ankle mortise. They are the most commonly due to eversion injuries. A less common fracture type is transversely oriented, extending from the medial corner of the ankle mortise superomedially to the medial cortex of the tibial metaphysis. This injury results from ankle inversion or adduction.

Posterior malleolus fractures are typically either avulsion at the site of the attachment of the posteroinferior tibiofibular ligament (due to external rotation) or fractures involving the joint surface due to impaction of the talus against the posterior aspect of the tibial plafond (secondary to forced plantarflexion of the foot).

An impaction fracture of the anterior tibial plafond with the foot in inversion and dorsiflexion is termed a pilon fracture. The radiographic appearance often involves comminution of the anterior plafond. These can be difficult to differentiate from trimalleolar fractures on radiographs and may require computed tomography (CT) for further evaluation.

The Tillaux fracture, an avulsion fracture of the anterior tibial tubercle, occurs with external rotation and abduction of the foot.[6] The fracture line extends vertically from the tibial articular surface proximally to the lateral cortex of the distal tibial metaphysis.

Fractures of the lateral process of the talus occur either due to ankle eversion and dorsiflexion, which leads to the impaction of the calcaneus against the lateral process of the talus or occasionally by ankle inversion. This has been termed the “snowboarder’s fracture” and is typically only seen on the AP ankle view.

Fractures of the posterior process of the talus are due to either forced dorsiflexion of the ankle, which causes an avulsion, or a chip fracture when the ankle is severely plantarflexed, causing the lateral tubercle to become wedged between the posterior lip of the tibia and the calcaneus. They occur more commonly at the medial tubercle and are best evaluated on the lateral radiographic view.

The above fracture patterns are most commonly seen in adults. Ankle fractures in children are relatively less frequent, but when they do occur, they tend to involve the epiphysis (Salter-Harris fractures), as the open physeal plate is a plane of weakness in any bone. The distal tibial epiphysis is the second most common epiphyseal fracture in the entire body, following the distal radial epiphysis.[7] Type I and type II fractures compose the majority of pediatric ankle fractures, and as with other parts of the body, nondisplaced type I and type II fractures are identified by a widened physis. Type III fractures may involve either the medial malleolus or the lateral aspect of the distal tibia growth plate at the anterior tubercle. Type III fractures tend to occur after the central portion of the physis has already fused, typically around age 12 to 13 years. Type IV injuries typically occur medially. Triplane fractures are complex fractures wherein the fracture line extends in a vertical plane through the epiphysis, a horizontal plane through the physis, and an oblique plane through the metaphysis. Lateral malleolar Salter-Harris injuries are less common and are most frequently type I injuries with minimal or no displacement.

Additional normal anatomic variants are important to appreciate, as they may be confused for fractures. Numerous accessory ossicles have been described throughout the foot and ankle and must be differentiated from fractures. Confusingly, many of these may occur at sites where fractures also occur, such as the distal tips of the malleoli and the anterior process of the calcaneus. The imaging clue that osseous fragments in these locations represent normal variant anatomy as opposed to fractures. These secondary centers of ossification are typically round and well-corticated along all margins, and the cortex of the adjacent bones is normal. Conversely, fractures tend to be irregular in appearance, with the absence of a well-defined cortex along the margin of the fragment and the adjacent bone. Avulsion fractures tend to be linear or curvilinear.

Plain Films

The standard radiographic analysis of the ankle includes three views: the anteroposterior (AP, also sometimes referred to as dorsoplantar or DP) view, the internal oblique (or mortise) view, and the direct lateral view. The AP view is used to evaluate the ankle mortise, though the lateral portions of the talus and tibiotalar joint overlap with the lateral malleolus, obscuring that area from view. The lateral process of the talus is best evaluated on AP ankle radiographs.[5][8] This view is also important for evaluating swelling about the medial or lateral malleolus. The mortise view better evaluates the talus and all margins of the joint space. In adults, all portions of this clear space should be symmetric and should measure no greater than 4 mm in width. The lateral view is valuable for evaluating ankle effusions. The lateral view should also include the proximal portion of the fifth metatarsal because of fractures in this region present with ankle pain.

The radiographic search pattern must include the evaluation of the soft tissues for swelling. Swelling about the medial or lateral malleolus is an important clue for underlying fractures or ligamentous injury. Ankle joint effusions may be identified by the presence of a soft tissue density in the expected position of the normal fat lucency anterior or posterior to the ankle joint. While many of the ligaments and tendons of the ankle and foot are not well-evaluated by radiography, the Achilles tendon is readily apparent. This tendon has an approximate average AP diameter of 6 mm. A size exceeding 8 mm may represent tendinous pathology from multiple sources, including tendinosis or tearing, post-surgical change, inflammatory arthropathies, or accessory muscles, among others.[9] A significant disruption of the tendon is also visible on radiographs.

Computed Tomography

Computer tomography (CT) is not typically used in the initial evaluation of the ankle. The utility of CT is to evaluate for clinically suspected radiographically-occult fractures, to evaluate displacement of fractures and associated dislocations, and to aid surgical planning. Retrospective reviews have demonstrated increased sensitivity of CT for evaluation of ankle and foot fractures as compared to radiography.[10] In particular, since talar and calcaneal fractures typically imply high energy trauma, a search for additional fractures with CT may be warranted in the emergent or urgent setting.[11][12][13][14]

CT is obtained in axial projections, and standard coronal and sagittal reformations are performed in soft tissue and bone algorithms. Real-time multiplanar reformations can be performed at the workstation, and 3D surface renderings can now be easily created to further assist the surgeon in planning. These features also allow a better evaluation of severe polytrauma patients, as patient positioning is not as important in this modality. Intravenous or intraarticular contrast is rarely administered in these examinations but may be used when contrast-enhanced magnetic resonance imaging (MRI) cannot be performed.

Magnetic Resonance

Magnetic resonance imaging (MRI) provides the highest soft-tissue contrast resolution of any imaging modality and is useful for the evaluation of soft tissue abnormalities and osseous lesions. There is little role for MRI in the emergent setting, as the high signal characteristics of edema and hemorrhage are nonspecific and may obscure other abnormalities. The exception to this is in the evaluation of a negative radiographic and CT evaluation of a suspected non-displaced fracture, stress fracture, osseous contusions, and osteochondral fractures, for which MRI is the gold standard. The principle utility of MRI lies in the evaluation of non-acute osseous and soft tissue lesions.

Routine MRI evaluation of the ankle is performed in the standard axial, coronal, and sagittal planes. There is significant variation in the exact name or sequences used to evaluate the angle, with imaging centers using variations in technique. However, there is typically some combination of T1 or proton density-weighted images, T2-weighted images, short inversion time recovery (STIR), or T2-weighted fat-saturated sequences. T1-weighted post-contrast sequences with or without contrast may also be added, depending on the clinical indication.

A thorough description of all pathologies within the ankle, which are evaluated by MRI, is beyond the scope of this article, but we will highlight common applications for evaluation of the ankle. The exquisite soft-tissue contrast resolution of MRI allows the evaluation of the tendinous and ligamentous anatomy of the ankle, which permits the characterization of sprains, tendinosis, tenosynovitis, peritendinosis, entrapment, rupture, and dislocation. Further, other soft tissue abnormalities such as sinus tarsi syndrome, impingement syndromes, compressive neuropathies, and synovial disorders are well evaluated with this modality. MRI has been shown to be the most sensitive and specific evaluation for musculoskeletal infections, including cellulitis, abscesses, and osteomyelitis. MRI has emerged as an important adjunct in the evaluation of inflammatory arthritides, for which it is the most sensitive modality to detect early changes. Finally, MRI is invaluable in the evaluation of soft tissue and osseous neoplasms, and other non-neoplastic masses.

Ultrasonography

Like sonography of most other joints, sonography of the ankle is performed less commonly than other imaging modalities. Sonographic evaluation is highly dependent on operator skill and is often performed directly by a musculoskeletal subspecialty-trained radiologist. The facility provided by sonography is the dynamic characterization of findings and the high spatial resolution. Sonographic confirmation and evaluation of soft tissue masses yield complementary information, but other imaging studies are often needed to fully characterize most masses.[15] Ultrasound is also invaluable in the use of image-guided interventions, such as biopsies.

Appropriate probe positioning to evaluate specific pieces of anatomy is crucial but beyond the scope of this article. A high-frequency (12 MHz to 17 MHz) linear transducer is used to maximize the evaluation of superficial structures.[16] The optimal technique for sonographic evaluation involves light pressure to avoid compressing vascular structures and limiting vascular flow. Conversely, a low-frequency (5 MHz to 9 MHz) curved array transducer is better for evaluating deep structures, and more transducer pressure can improve evaluation by decreasing distance to the finding. Static images in orthogonal planes should be obtained to determine any mass's size and evaluate for vascularity. Cine images are valuable for evaluating large masses, demonstrating the origin of the mass, and demonstrating spatial relationships to adjacent structures. A dynamic evaluation provides additional characterization.[17] Compression of the abnormality is useful to evaluate for swirling of contents within a complex cystic lesion, as may be seen in a ganglion cyst. The movement of adjacent structures can aid in determining the origin of the lesion, as in the case of a giant cell tumor of the tendon sheath, as this lesion would not move while moving the tendon itself during flexion or extension of the joint.

Though a discussion of ultrasound physics and artifacts is beyond the scope of this article, it must be mentioned that excessive system gain and near field reverberation artifacts, among others, need to be minimized by adjusting the focal zone, scanning plane, and system gain. Some ultrasound “artifacts,” such as posterior acoustic enhancement or shadowing, provide valuable information, demonstrating a lesion as cystic or solid, respectively.

Ultrasound is not useful for the evaluation of most osseous structures and lesions because of the dense posterior acoustic shadowing deep to the highly echogenic cortex. The sonographic appearance of normal anatomic structures has been previously described. We will limit this discussion to the appearance of muscles, tendons, and ligaments for the sake of brevity, as these structures are the most commonly investigated structures in the acute or traumatic setting. Muscles appear hypoechoic, and linear hyperechoic striations corresponding to fascial lines are often seen. Tendons are hypoechoic and demonstrate a linear fibrillary echotexture. Ligaments appear similar to tendons, though the linear striations are more compact than those of tendons.[18] Any disruption of this well-ordered appearance, especially with an appropriate traumatic or overuse history and physical examination, is suggestive of an injury to these structures. In particular, evaluation for tendinosis, tear, or rupture of the Achilles tendon is easily performed with ultrasound due to its superficial location. In the event of an acute or subacute injury to one of these structures, an adjacent complex fluid collection is suggestive of a hematoma. Note, however, that differentiation of a hematoma from a hemorrhagic mass is not possible with ultrasound, and such a finding should be followed to resolution and biopsied if it grows or persists.

Nuclear Medicine

Scintigraphic evaluation of the ankle is not indicated in the acute setting. The utility of nuclear medicine techniques lies in the evaluation of subacute or chronic ankle pain, for which the radiographic examination was negative. For the sake of brevity, we will defer extensive discussion of the technique of the examination, which has already been presented.[19][20] The most common examination is a Tc-99m labeled bone scan and may be performed with 1, 3, or 4 phases. Following venous injection of the radiotracer, the first phase, performed over 60-90 seconds, is the flow or angiographic phase, which demonstrates perfusion of the area. Differences in radiotracer activity are due only to differences in the flow. 5-10 minutes after injection, the blood pool phase is acquired. Differences in activity in this phase are caused by differences in flow and in capillary dilatation, which is nonspecific and secondary to inflammation. The third delayed phase is acquired 2-4 hours after injection and provides the greatest signal-to-noise ratio, as soft tissue uptake is minimized following normal excretion of the radiotracer. Increased radiotracer activity is due to a combination of blood flow and osteoblastic activity. A fourth phase can be added for greater specificity and is usually performed 24 hours after injection. This phase can be helpful in cases of persistent soft tissue radiotracer activity obscuring possible osseous activity, as may be seen with soft tissue infections or in patients with peripheral vascular or renal disease, which prevents prompt radiotracer excretion or if there is a concern for osteomyelitis.

A small degree of uniform, symmetric osseous radiotracer activity within the foot and ankle is normal. Increased radiotracer activity in the delayed phase(s) is nonspecific. It may be due to numerous pathologies, including degenerative changes, fractures, infection, avascular necrosis, malignancies, and non-malignant osseous diseases such as Paget disease, fibrous dysplasia, osteoid osteoma, and complex regional pain syndrome. A similarly large differential for focal soft tissue uptake includes several acute and chronic diagnoses. It is the combination of the imaging findings, the distribution, and the clinical history, which leads to an appropriately narrowed differential diagnosis.[21]

The conventional bone scan is a collection of planar scintigraphic images that present a two-dimensional picture of the imaged region, analogous to radiograph. Alternatively, the scintigraphic data can be obtained using single-photon emission computed tomography (SPECT) with the imaging cameras in a gantry that rotates about the patient, analogous to a CT, providing a three-dimensional image of the radiotracer activity. This can be performed concomitantly with a CT (SPECT-CT). The data co-registered so that the metabolic activity of the radiotracer can be overlaid with the anatomic information from the CT to improve specificity and localization. Newer nuclear medicine techniques for the evaluation of osseous lesions include positron emission tomography (PET) combined with CT (PET-CT) using F18-NaF. These studies have the advantage of a better target to background signal ratio but have increased radiation dose and cost.

Two additional important scintigraphic studies that may be utilized to evaluate osteomyelitis are the tagged white blood cell (WBC) scan and sulfur colloid bone scan. WBC’s (primarily neutrophils) taken from the patient are tagged with In-111 and injected into the patient. The patient is then imaged 24 hours later. As with any infectious or inflammatory process, WBC’s will localize to a site of inflammation. If an area of increased activity is identified, this may then be followed by the sulfur colloid study. Tc-99m labeled sulfur colloid will localize to the marrow in a normal patient due to reticuloendothelial activity of the marrow. In the setting of osteomyelitis (or any other marrow replacing process), sulfur colloid will not accumulate in the marrow. Therefore, a spatially incongruent combination of images that have increased activity in the bone on the WBC study and decreased or no activity on the sulfur colloid study is consistent with osteomyelitis.[22]

Angiography

As in other parts of the body, angiography of the ankle can be performed by conventional angiography, CT angiography (CTA), or MR angiography (MRA). Conventional angiography using digital subtraction angiography (DSA) remains the gold standard for the evaluation of arterial anatomy. Conventional angiography is invasive, however, requiring direct arterial access. A benefit is using DSA is the real-time nature of the examination, allowing selective and sub-selective catheterization of distal arteries and the ability to intervene at the time of the examination. Additionally, DSA has the highest spatial resolution of these modalities, allowing for the detection of small arterial injuries below the limit of resolution of CT or MR.

The most common indication for these studies is vascular trauma and is most commonly performed with CT. CTA has been shown to have near 100% sensitivity and high specificity for vascular trauma to the ankle, which is less expensive and less time-consuming than conventional angiography and can be performed concomitantly with the trauma evaluation.[23] Further, in those patients who will undergo operative management, the CTA can provide valuable information for planning the vascular intervention.[24] Images are typically acquired in the arterial phase only, but additional phases may be added as needed. In particular, a delayed phase may be added if hyperdensity adjacent to a vessel is seen, and differentiation between a contained hematoma and active hemorrhage is needed.

Vascular injuries may present as intraluminal thrombus, dissection flap, non-opacification of a vessel due to occlusion, or extravasation of contrast correlating with active hemorrhage from mural disruption. A dissection flap may be either linear or semilunar in the configuration. These must often be differentiated from nontraumatic findings such as calcified and noncalcified atheromatous plaque in the arterial wall. Abrupt vessel narrowing with distal vessel opacification may suggest either intimal injury, external compression, or vasospasm. Vessel narrowing may be subtle and difficult to differentiate from the normal tapering of distal vessels.[25] A more organized extravascular contrast-containing sac connected to the vessel lumen is consistent with a pseudoaneurysm. Early venous filling on an appropriately-timed arterial phase, CTA suggests arteriovenous shunting, possibly from a fistula. In an acute post-traumatic fistula, the veins will not be significantly increased in caliber. Dilated contrast-opacified veins suggest a more chronic nature.

MRA is not indicated in the acute setting but is useful in the evaluation of chronic peripheral vascular disease such as in diabetes, especially in those patients for whom exposure to iodinated contrast or ionizing radiation is a concern. Numerous techniques have been investigated for MRA, including gadolinium-enhanced phase-contrast and non-contrast time-of-flight (TOF), flow-sensitive dephasing, and steady-state free precession techniques. Sensitivity and specificity for all of these techniques are high.[26][27]

Patient Positioning

The AP and lateral views may be obtained with or without weight-bearing. For non-weight bearing images, the patient is seated on the table with the hip and knee flexed and the ankle in a neutral position or supine with the entire leg straightened on the table. AP images are obtained by directing the x-ray beam from the dorsum of the ankle to the plantar surface, with the image receptor beneath the sole of the foot. Internal oblique images are obtained by internally rotating the ankle 15–20 degrees and directing the x-ray beam in a dorsoplantar direction similar to the AP view. Lateral images are obtained by directing the x-ray beam from left to right, with the image receptor medial to the ankle. Weight-bearing images are obtained similarly, though with the patient standing. Typically, a special stand with steps is used, with a slight cut into the top step used to hold the x-ray cassette.

The gravity stress view is obtained using a cross-table projection of an externally rotated foot and ankle, with the affected side-lying upward. The ankle is allowed to lie distal to the edge of the table so that the weight to the foot provides the gravity stress. The talar tilt stress view is obtained by maintaining the talus in inversion by holding the talus and tibia while obtaining a dorsoplantar projection. Finally, the anterior drawer stress view is obtained by allowing the leg to suspend from the edge of a table while the ankle lies in a gravity-assisted plantar flexion, and the foot is pulled while stabilizing the tibia.

Multiple specialized radiographic views of the ankle have been described for specific purposes. The external oblique is used to view the medial column of the foot, navicular, medial cuneiform, first metatarsal, and its articulations and is obtained similar to the internal oblique view, except that the ankle is in 45 degrees of external rotation. The Harris view better evaluates the calcaneus, middle facet of the subtalar joint, and sustentaculum tali. This is obtained with the patient standing and beam angled approximately 45 degrees toward the midline of the heel. Broden views are superior for imaging the subtalar joint and are obtained with the patient in supine, knee slightly flexed, and supported by a sandbag, with the foot in neutral dorsiflexion. The leg and foot are internally rotated 45 degrees, and images are obtained in multiple degrees of cephalic tilt, as necessary. The Canale view is useful for assessing the degree of comminution and displacement of talar neck fractures and is taken with the foot plantarflexed and pronated to 15 degrees. Finally, the Saltzman view has been used to determine the relationship of the hindfoot with the leg to characterize complex hindfoot malalignments and is obtained both with and without weight-bearing, with the beam oriented posteroanterior and craniocaudally, tangential to the hindfoot.

Patient positioning for CT is less important due to the cross-sectional nature of the study and typically easily performed multiplanar reformations of the obtained images. Additionally, the real-time image acquisition nature of ultrasound and angiography obviates the need for special patient positioning in these situations. Conventionally, images provided to the interpreting provider are presented in the standard three anatomic planes to aid in recognition and interpretation.

MRI evaluation of the ankle is typically performed with the patient supine and the foot in approximately 20 degrees of plantar flexion. The plantar flexion is important to decrease MRI artifact (magic angle), to accentuate the fat plane between the peroneal tendons, and to allow better visualization of the calcaneofibular ligament.[1]

Clinical Significance

While all fractures are important to identify, the most concerning for the image interpreter are the ones most likely to be missed. Failure to identify such fractures can lead to delayed diagnosis and suboptimal management, including growth plate disturbance (in pediatric patients), persistent pain and instability, fracture nonunion, and accelerated osteoarthrosis.

When no fracture is identified, it is helpful to look at specific injury sites, as opposed to a general survey of the ankle. Perhaps the most commonly missed ankle fracture is the anterior process of the calcaneus and may only be seen on the lateral radiographic view.[28] Nondisplaced medial or lateral malleolus fractures may only be seen on mortise views. As mentioned above, fractures of the proximal fifth metatarsal may be seen only on lateral ankle views and may not be apparent on foot radiographs. These fractures may not be clinically suspected, as the presentation is lateral ankle pain, and physical exam may be more suggestive of an ankle sprain. Fractures of the talar dome, termed osteochondral defects (OCDs), may initially be missed on the first evaluation of the ankle and should be sought out if other fractures are not identified. These typically occur at the medial and lateral corners of the talar dome. Similarly, fractures of other tarsal bones should be sought out. Avulsion fractures can occur at midfoot and hindfoot tarsal bones.

An important consideration is a case where a fracture is seen involving the posterior malleolus or a displaced fracture of the medial malleolus without an identified fracture of the imaged distal fibula. If there is a concomitant widening of the syndesmosis or medial clear space, this implies a syndesmotic ligamentous injury. The line of force will be transmitted to the proximal fibula. As such, the image interpreter should recommend AP and lateral views of the tibia and fibula to evaluate for proximal fibula fractures, the so-called Maisonneuve fracture. The more proximal fracture may not be clinically suspected because patients will typically complain of ankle pain and not be aware of the more proximal injury.

Radiography remains the principal imaging evaluation of the ankle. Advanced imaging of the ankle with ultrasound, CT, MRI, ultrasound, nuclear medicine, and angiography is reserved for radiographically occult fractures, soft tissue abnormalities, vascular injuries, and other specialized considerations.